Polyoxometalates as catalyst precursors for reverse water gas shift

The reverse water gas shift (RWGS) was studied in the presence of catalysts based on Keggin-type polyoxometalates (POMs) such as heteropoly acids H₃PMo₁₂O₄₀ and H₃PW₁₂O₄₀ and their Co(II) and Ni(II) salts. Under reaction conditions, molybdenum POMs decomposed to form Mo oxide species, which exhibited high activity, selectivity and resistance to deactivation in the RWGS reaction. The catalysts formed from H₃PMo₁₂O₄₀ supported on SiO₂, TiO₂ and γ-Al₂O₃ produced CO with 100 % selectivity at 31–33 % CO₂ conversion at 600 °C and a CO₂:H₂ molar ratio of 1:1 (cf. 39 % equilibrium conversion). H₃PMo₁₂O₄₀/SiO₂ was more active than MoO₃/SiO₂ with the same Mo loading, revealing a phosphorus enhancement effect on the activity of molybdenum species. This effect can be attributed to the influence of phosphate on the dispersion of the Mo oxide active phase. This is supported by STEM, which shows different morphologies of Mo oxide species in H₃PMo₁₂O₄₀/SiO₂ and MoO₃/SiO₂ catalysts. Chemical looping and H₂-TPR of fresh and spent Mo catalysts show that the reaction occurs through the redox mechanism involving the reduction of Mo(VI) oxide species to Mo(IV) with H₂, followed by reoxidation of Mo(IV) with CO₂ to form CO (reverse Mars–van Krevelen mechanism). Contrary to the Mo catalysts, tungsten POMs, exhibited no RWGS activity, which can be attributed to their resistance to reduction.